Electric furnace



Dec. 9, 1930.

A. IMBERY ELECTRU: FURNACE 8, 1928 2 Sheets-Sheet l Filed Oct C MWATTORN EY A. IMBERY ELECTRIC FURNACE Dec. 9, 1930.

8, 1928 2 Sheets-Sheet 2 Filed OCT..

Patented Dec. 9, 1930 UNITED STATES- PATENT GFFICE .ARTHUR IMBERY, FHALIFAX, ENGLAND, ASSIGNOR TG GLOBAR CORPORATION, 0F NIAGARA FALLS, NEWYORK, A. COB-PRATION OF NEW YORK ELECTRIC FURNACE Application ledOctober 8, 1928, Serial No. 310,981, and in Great Britain October 10,1927.

This invention relates to an electrical furnace containing a pluralityof heating elements and a lurality of muil'les, in Whose in,- terior itis esired to obtain a temperature which is made substantially uniform inthe interior of the portions of the mullle subjected to heat radiationsby (l) the symmetrical distribution of the mules With respect to theheating elements and by the screening effect of the highly conductingWalls of the inutiles which smooth out temperature differences due toWant of uniformity of temperature of the heating elements and theirvarying positions with respect1 to different parts of given muflles. Myinvention is useful for example in the heat treatment of mining drillsWhere a uniform temperature at the working ends is desired.

One difficulty in maintaining the temperature desired for annealingr andsimilar purposes has been the diiiiculty of: maintaining a plurality ofinutiles at the same temperature owing to non-uniform heating as isparticularly the case in combustion furnaces. Electrie furnaces are moreeasily regulated in this respect than combustion furnaces, but Wheremetallic heating elements are used in commercial practice they' areliable to burn out at points of high local resistance with the resultthat operations are interrupted during the period of repair andreplacement. The upper temperature limit at which metallic resistors canbe used is about H000 (lion account of the strong tendency to oxidationin metals that can be used in commercial practice. The teinicratures ina protected muilie heated by metallic resistors Will of course besomewhat l a inutile of large dimensions is used the temperatures ofdifferent parts of the 49 mulie Wall Will var considerably on account oftheir dierent distances from the resistor elements and the length ofconducting,l path between a portion of the wall subjected to highertemperatures and another portion of thy. Wall subjected to lowertemperatures.

l propose to overcome these difficulties by usingv a plurality ofnouilles et small diameter in which (l l the thickness of the Wallis larje with respec to the diameter and (2) the Wa ls 5@ are made et highlythermal conductingL material such as an iron alloy capable ofwithstanding temperatures up to 1100 C. or silicon carbide which willwithstand temperatures up to 1400 C. under the furnace conditions to bedescribed. The thermal conductivity of silicon carbide is about 0.03calorie/c'm.3/C./sec at 1000o C. This value, however, depends on thetemperature and bonding material. A. value as low as 0.0127 has beenpublished.

I propose also to use non-metallic resistors of large size which willremain rigid at temperatures up to about lll-(lilo C. Resistors ofgraphite or some other form of carbon may be used. These resistors arehowever subject to rapid oxidation at MOGo l. and graphite is of toohighconductivity for use in short rods of the size desirable for thefurnace which I describe below. Silicon carbide is better suited for thepurpose on account oit its greater electrical resistivity and itsgreater resistance lo oxidation. Silicon carbide may be ceramicallybonded into rods Which are very strong and resistant to oxidation athigh temperatures. Such resistors can be used to attain and maintaintemperatures up to lll-00 C. 'l'lie non-metallic heating elements areplaced in chambers or ovens of refractory material and :ire arranged insuitable proximity to tubes which er; tend into or through such ovensand receive the bars t0 be treated. These tubes may be made of siliconcarbide or elimine iron containin about 27% chromium, or otherl'nateriai which Will stand the temperature required. The furnace may beused on either alternating or direct current circuits. The electricalconnections to the non-metallic heating elements are effected by metalalloy tern'iinals of a lower electrical resistance than the.poinmetallie resistors. These metal terminals may be so placed that thehout is practically all developed by the electric current `wi '1 theinutile or adjacent theV inner surface of the Inutile wall. The ends of'the non-metallic. resistors may also be enlarged or (it oi siliconcarbide) impregnated with silicon te reduce the end resistance ot theresistor rods,

My improved fi :nace is .illustrated by the accompanying drawings inwhich:

, Figure 1 is a longitudinal vertical section taken on the line 1-1 ofFig. 2.

Figure 2 is a vertical section taken on the line 2-2 of Fig. 1;

Figure 3 is a vertical section of a modied arrangement in which theresistor elements extend in two mutually perpendicular directions andFigure 4 is a section taken on the line 4-4 of Fig.

In the drawing the reference character A indicates non-metallic heatingelements, B the chamber or oven, C the tubes or mulles for the receptionof the drills to be heated. The oven or chamber B is constructed ofrefractory material b. This may be of tire.

clap or silicon carbide brick. In the example shown there are threesuperposed rows of non-metallic heating elements A which extend in alongitudinal direction through the oven. These elements are shown asmade in the form of rods and are supported by alloy terminals a whichare stationary at one end and maintained in Contact with the nommetallicresistor rods at the other end through the action of springs a toprovide for expansion and contraction through heating and cooling Thesaid alloy terminals are as already indicated of lower resistivity thanthe non-metallic rods, so that the heat developed in them is not sogreat as in the resistors. The oven or chamber so formed together withthe special alloy terminals may be embedded in a brick work or othersetting of refractory materials 291 mounted in a metal casing b2supported on a stand ba. The refractory materiai hl may be a porous clayrefractory to give the Walls good insulating qualities. Extendingtransversely between the rows of nou-metallic heating elements are thetubes C which also extend through the brick work or other refractorysetting and through the outer metal casing and are there provided withhinged Haps or doors c which normally close on the tubes by gravity. Thebore of each tube is closed at its central portion c1 to provide a stopVtor the drills, and the length of the tubes together with the thicknessof the brickwork setting is such that it is only the centrai portions ofthe alloy tubes that are heated to the maximum temperature with theresult that loss of heat by radiation and through the tube openings (inwhich the driiis are inserted) is reduced to a minimum. In alternatingcurrent circuits a transformer may oe employed with a number ofsecondary taps in order to provide an adjustable voltage across theheating elements to compensate .for auf change in resistance that maytake pia the high operating temperatures to material is subjected andgive an life to the heating elements 'with a c iseouent lowermaintenance cost. In som.Vx an induction regulator .may be empioyedinstead of the transformer to give the desirable adjustments of voltage.The equipment may be provided with automatic temperature control toenable uniform results to be obtained even when the equipment isoperated by unskilled labor.

In the modification shown in Figures 3 and 4 two sets of mutuallyperpendicular resistor rods are used.` Here `certain diaphragms of thetubes C are shown as lying in the plane of one set of resistor rodswhile each diaphragm is also symmetrically placed with respect to fourresistor rods. The tubes may be made of chrome iron (27 per centchromium) and with walls about 1/2 inch thickness in a tube of 2 inchesmean diameter so that temperature differences due to one part of adiaphragm being nearer one rod than another or due to uneven temperaturedistributions in the rods are smoothed out by the comparatively easyconduction of heat in the metal.

To get some quantitative idea of the effect .of the thickness of theWalls of the tubes as compared with their distance from the axis we citethe following example:

In the case of a cylindrical tube in which the mean radius of the wallsis 3 centimeters and their thickness is 1 centimeter the thermalresistance from a square centimeter outer surface exposed to maximumradiation from a resistor rod (for example directly above the unit area)to a unit area on the outer lateral surface (90 from the first` unitarea) would be about four times the thermal resistance to flow of heatdirectly across the wall. If now, the mean radius being kept the same,the thickness of the wall be reduced to l millimeter, the above ratiobecomes about 100 times as great, because the length of path for flowdirectly across the wall is reduced t0 one tenth and the cross-sectionof the path vfrom one unit area to the other unit arearemoved ninetydegrees is reduced to one tenth. Both of these effects work together toincrease the ratio referred to. This example shows the larje stiect thatthickness of the walls of the tu es have in evening out the temYperatnres of the walls,q different portions of which are exposed todifferent intensities of radiation.

The central portions of the inutiles are shown supported in the liningof the oven. The outer portions are not shown as integral with thecentral portions for the reason that l propose to make the outerportions of the tubes (i of firewclay or some other material of thermalcoi'uiuctivity ktess than 0.005 calorie/'cm/OC./sec [n this Way thelongitudinal conductim ot heat from the middle portion of given inutilecan be greatly reduced and the miitormity of temperature ot' thecent-ral portion increased. My furnace vis also well adaptcf torcontinuous operation so that the inner portions of the refractory wallsare maintained at a temperature sub stantially the same as that of theoven wall b, especially when that is made of silicon carbide. In thesevarious ways the uniformity of the temperature of the muliles isincreased.

The furnace which I have described is particularly adapted to giveuniform temperature conditions for the cutting ends of drills which areused in large numbers and are of uniform material. The furnace as shownin Figures 1 and 2 affords accommodation for forty rods with the use ofany six non-metallic resistors. The drills are protected from theintrusion of foreign material (while being heated) by the surroundingtubes C. These surrounding tubes have their middle portions each atequal distances from four resistor rods so that the medial diaphragms ofeach of the forty tubes are likely to acquire substantially the sametemperature with the possible exception of those nearest the oven walls,subject of courser also to possible variations in the resistor rods frompoint to point. By making the rods of large diameter and with carefulattention to uniformity of material in the manufacture good results maybe obtained.

I claim:

1. In an electrical furnace an enclosed chamber, a plurality of rigidresistors each extending substantially across said chamber, and aplurality of muiiles also extending across said chamber, the middleportion of eachmuiile being equidistant from a plurality of saidresistors distributed around it.

2. In an electrical furnace for the heat treatment of metal drill rods,an enclosed chamber, a plurality of conducting tubes extending acrosssaid chamber and constituting heating chambers for said drill rods, anda plurality of non-metallic heating elements each extending across saidchamber and placed adjacent said conducting tubes.

3. The furnace described in claim 2 in which the conducting tubes areall parallel to each other and in which the heating elements arearranged at right an les to the conducting tubes, the middle portion ofeach conducting tube being centrally placed with respect to the nearestheating elements.

4. The furnace described in claim 2 in which the heating elements arearranged equidistant from each other in parallel rows and in which theconducting tubes are arranged transversely to the heating elements,diaphragms closing the middle portions of the bore of each conductingtube to correctly position the ends of two drills under treatment, saiddiaphragms being centrally placed with` respect to all of the nearestconductors.

5. In an electrical furnace for heating a plurality of drills, heatingchambers for the drills having central diaphragms, resistor rodsdisposed transversely to the heating chambers, each diaphragm beingcentrally located with respect to four resistor rods.

.sides of said tubes, diaphragms in each of said tubes locating thecentral portion of the heated tube and acting as a stop to position -twodrills under heat treatment.

7. An electrical furnace containing a plurality of muflles insubstantially arallel arrangement, a set of unitary, sel -sustainingresistor elements substantially at right angles to said muilles andextending between terminals of higher conductivity, and a second set ofsimilar resistor elements which extend substantially at right angles tosaid muflles and to said first set of resistor elements.

8. The electric furnace described in claim 7 in which each metal tubehas a central diaphragm'symmetrically situated with respect to aplurality of resistor rods, said diaphragms serving as stops for drillends to be heat-treated.

9. In an electrical furnace for heatin a plurality of objects, a mainheating cham er surrounded by refractory walls; tubes of good thermallyconducting material for supporting the objects under treatment withinthe main heating chamber, whereby the objects in any one tube may bereplaced without disturbing materially the objects in the remainingtubes; and a plurality of rigid resistor elements each of which extendsacross said main heating chamber, said resistor elements being onopposite sides of said conducting tubes.

In testimonyl whereof I aix my signature.

ARTHUR IMBERY.

